1. Spin-Peierls Effect on Frustrated Spin Systems
Cheng Long Jia and Jung Hoon Han
Department of Physics, Bk21 Physics Research Division, and Institute for Basic Science, Sungkyunkwan University, Suwon , Korea
----Lattice-Coupled AFM Model----
Hexagon organization of Spins in Pyrochlore i j ui uj
Rij
Spin and lattice are coupled through magneto-striction effect
Lattice displacement related to local spin-spin correlation by
Arrangement of 4 types of hexagons
Arrangement of 3 types of spin directions
----Partial Valence-Bond-Solid----
----Application to Frustrated Lattices----
Experiments on YMnO3
Triangular and Kagome Lattice
NO LATTICE DEFORMATION!
S Lee et al. PRB(R) 2005
Expansion of Mn triangles (red) at the onset of magnetic order
IDEA: Spin-Lattice Coupling –induced VBS ?
Pyrochlore Lattice
Magnetic order doesn’t work, but valence-bond order might.
Structure
Partial VBS ORDER
Tetrahedron as a building block
Ground state condition
At most 4 VB’s can form
for coordination = 6
Some links must lack a VB and expand.
for each tetrahedron
Macroscopic ground state degeneracy in
pristine Heisenberg model
Ground State
Experiments on ZnCr2O4
Schwinger boson mean-field theory calculation (cond-mat/ ) supports a ground state with lattice distortion and modulated bond amplitudes, consistent with our picture of partial VBS
Spins on Cr3+(S=3/2) order antiferromagnetically
accompanied by cubic-to-tetragonal distortion
(S.H. Lee, PRL, 2000)
Below Tc
Neutron scattering structure factor consistent with hexagon spin cluster, i.e. neutrons scatter off a hexagon cluster of spins
(S.H. Lee, Nature, 2002)
Above Tc
A New Excitation
Lifting of Degeneracy through Spin-Lattice Coupling
Tetrahedron Unit
(Tschernyshyov et al. PRL 2002)
Hexagon Unit
(Jia et al. PRB 2005)
PVBS
Spin Wave
Comparison